Time proportional control systems that are condition responsive are known. One of the major applications of this type of condition responsive control system is in the control of heating and cooling equipment. The present invention is generally applicable to any type of condition control system that utilizes condition responsive time proportional control, but will generally be described in terms of a thermostatically controlled system or a thermostat.
A thermostat typically uses thermal anticipation to obtain a better system performance. This anticipation reduces the dependence on the ambient space temperature to activate the thermostat between its "on" and its "off" condition. Various means are used to obtain the anticipation heat, but all of these are thermal and are, therefore, subject to different air flows that exist in different installations. If the actual air flow over the thermostat in a particular application is greater or less than the air flow the thermostat was designed for, the actual temperature rise of the sensor due to the anticipator will be reduced or enhanced. This will result in less than optimum performance. A similar effect will occur if the air flow changes from time to time in a given installation. If the air flow is constant, the anticipator can be readjusted to bring back optimum performance, but in changing air flow conditions, no one setting will be optimum. It should be noted that in most thermostats, a change in the characteristics of the anticipator will also change the entire system droop.
In an electronic thermostat, anticipation can be achieved electronically. This has the advantage of not being affected by air flow and thus eliminates all of the problems associated with thermal anticipation as noted above. One method of obtaining this type of anticipation is the use of a resistor and a capacitor charge and discharge arrangement as part of the negative feedback in an electronic amplifier while using a fixed positive feedback. This type of electronic anticipation is injected as a negative feedback made with a single order time constant. For proper operation, this time constant may need to be in the order of sixteen minutes. To obtain this type of time constant with a single resistor-capacitor arrangement requires high resistances and a very low leakage, large capacitor. This arrangement makes obtaining this type of electronic anticipation impractical. The size of the resistor and capacitor would place a burden on the cost of the device, and on the physical size of the thermostat itself.
To obtain the desired time constant of approximately sixteen minutes, a relatively small capacitor and reasonably sized resistors can be used thereby obtaining the relatively fast cycling rate in the time proportional control circuit. This relatively fast cycling rate can then be directly counted. If a counter is allowed to count up at a given rate during the "on" time of the anticipation, and another counter is allowed to count up at the same rate during the "off" time, we would have a digital representation of the "on" and the "off" time periods for the desired operating condition (this is the actual deviation from the set point of the room temperature). The sum of these two counters is the cycling period. this type of information gives a complete description of the cycling pattern of the system for a constant input of a given magnitude. If the average room temperature and the set point remain constant, we would then let the cycling pattern continue, but no longer allow the counters to count up. Each time the "on-off" action of the comparator or electronics occurs, the time counter would be reduced by one count. When the counter reaches zero counts, the system would turn "off". The "off-on" action of the comparator or electronic amplifier would then start to count down the "off" time counter. When the "off" time counter reaches zero, this system would then turn "on" and the counter would be allowed to count up at the given rate. This multiplies the "on" and "off" period of the number of counts stored in the counter. Since the basic "on" and "off" periods are determined by a constant, the constant also effectively, multiplies by the same constant. To keep the system closer to the actual operating conditions, the "off" period counter can be updated each time the "on" period counter is counted down. Similarly, the "on" period counter can be updated each time the "off" period counter is counted down. As thus described, the system would work well as long as the comparator is cycling. However, if a set point change is made, or the deviation from the set point is such that the cycling stops, there is a possibility that the control can go out of "phase". That is, that furnace can be "on" when it should be "off", or the opposite can occur. Therefore, some means must be provided that will sense when these conditions occur and force the output into the proper state. One way would be to use two level detectors which would force the output into the proper state when the deviation from the set point is greater than the maximum anticipation signal or when the deviation is effectively negative. This method would involve a very critical calibration. An expanded time constant control system utilizing the up-down counters has been previously disclosed. That system utilizes a time proportional control system coupled with an up-down counter and a pulse generating means that had a signal combined in the counter to expand the time constant. The use of an up-down counter entailed certain complexities that may be avoided or simplified.
One simplification is in the use of a time proportional circuit utilizing a relatively small capacitor and resistors, and a rapid cycling rate. The rapid cycling rate is then sensed by a unidirectional counter that forms part of the counting means. The unidirectional counter in one simple form, is a ripple counter. The cycling rate of the time proportional control is combined with the pulse rate of a pulse generating means. This arrangement utilized a readily available type of digital counter. It further had the advantage that the system could never go out of synchronization with the state of the condition being responded to even if there was a sudden change in the condition or a sudden change in the set point of the condition responsive system. That type of system, however, could have an undesirable delay that occurs when the set point is suddenly changed to a value which would be outside of the proportional band of the system. The system would than have to wait out a rather long period of time until the inputs and the outputs of the control system are brought into proper correspondence with one another.